This invention relates to a plastic disc brake piston with a fluted core design for superior structural strength, crack resistance, and improved heat dissipation, without sacrificing contact surface area.
Although the principal area of application of disc brakes is for use in motor vehicle, particularly automotive, braking systems, the present invention can be useful whenever braking force is applied to slow or stop an object in motion, such as an aircraft, a rotor, an auxiliary drive, or other similar devices. Therefore, although the present invention will be described for illustrative purposes in an automotive context, it is not intended in any way to so limit the present invention, and uses of the present invention are contemplated for other purposes than automotive braking systems.
In recent years a high priority has been placed upon achieving significant weight reductions in the design of new automobiles. This objective has been given even greater impetus under the impact of fuel shortages and the continuing need for fuel and energy conservation. Accordingly, considerable developmental work in recent years has directed attention towards materials substitution for various automative parts, with a trend toward use of more non-metal parts having been clearly established. Illustration of the extent to which metal parts have already been replaced by parts made of lighter materials can be found in an article appearing in Autoproducts, December 1974, pgs. 4 to 5, where it is stated that the average 1975 model car contains more than 150 pounds of plastic. The particular component with which the present invention contemplates replacing a metal with a plastic part is located within a vehicle braking system, more particularly, within a disc brake caliper assembly.
It has been recognized that for many vehicle applications, a disc type brake system offers considerable advantages over internal expanding brakes. Thus, for example, a disc brake is more readily responsive to the actuation of the master cylinder, provides a firmer engagement of the rotating surface, and requires a smaller stroke of the master cylinder piston than most internal expanding brakes. The disc brake generally comprises a rotating brake disc coupled with the vehicle wheel or axle and has a pair of annular brake faces lying in planes transverse to the axis of rotation of the disc. Flanking the disc and juxtaposed with these annular braking surfaces over only a fraction of this surface are a pair of brake shoes of segmental configuration which engage only limited regions of the braking faces. Such brake shoes are mounted in a yoke which is connected to a stationary portion of the vehicle, e.g. the axle housing or chassis, and are forced by respective wheel brake pistons against the disc. Such pistons are located in wheel brake cylinders formed in the yoke and are connected by hydraulic lines to the master cylinder. In other variants, only a single wheel brake cylinder is provided and either the disc or the yoke is axially shiftable to bring the opposite face of the disc into engagement with a brake shoe fixed on the yoke when the piston forces its brake shoe against the disc. U.S. Pat. No. 3,442,356 to Hahm describes a low-noise hydraulic spot-type disc brake with a piston fitted with a sleeve of low friction, wear and corrosion resistant material, such as stainless steel, having a polished internal surface, of synthetic resin such as polytetrafluoroethylene, or other suitable material. No mention is made in Hahm of use of other than the conventional metal materials for construction of the piston itself.
U.S. Pat. No. 3,599,758 to Bishop shows a disc brake assembly of the sliding caliper type, illustrating the construction of a typical disc brake system and the relationship of the disc brake piston with the disc brake cylinder housing.
Further illustrations of disc brake assemblies employing pistons of conventional construction can be found in Haraikawa, U.S. Pat. No. 3,804,212; in Swift, U.S. Pat. No. 3,361,229; Dowell, U.S. Pat. No. 3,094,193, showing an application of disc brakes to aircraft; Baxendale, U.S. Pat. No. 3,896,907, showing an application to large trucks or tractor-trailer combinations; and Erickson in U.S. Pat. No. 3,115,952, showing a vehicle disc brake and piston assembly.
Conventional disc brake pistons have been manufactured of steel by a cold extruding process or draw forming steel plate, followed by a grinding process and by plating with a layer of chromium to a thickness which will decide the final tolerance. Due to the fact that tolerances necessary are extremely close, the chromium plating process is very deliberate, and production of a considerable amount of scrap is unavoidable. Moreover, chromium-coated steel pistons are subject to corrosion problems which in servicing necessitate buffing the metal with an abrasive and then reinserting the piston. Another problem encountered with plated steel pistons is rapid heat transfer to the hydraulic fluid, the result of high thermal conductivity through the metal. The weight of a steel disc brake piston used on a full-sized automobile can be typically about 790 grams, while the weight of the piston of the present invention, substitutable for such a steel version, is about 350 grams. Weight savings are multiplied by the number of braking units used in a vehicle which may be two units for some models of automobiles, four units for others, and an even greater number for tractor-trailer or aircraft use.
A difficulty which, however, has been encountered with plastic disc brake pistons in the past has developed under conditions of prolonged usage when the part has been exposed to service operation where wide temperature variations and repeated mechanical stresses typically are encountered. When the prior art plastic disc brake piston is exposed to service conditions where wide temperature variations are encountered, small cracks can appear on the pressure bearing face, radiating from the inside wall outward. In FIG. 1 a plastic piston of the type described in Autoproducts, December, 1974, demonstrates this condition where the open end of the prior art plastic piston has been exposed to test apparatus temperatures of 500.degree. F.
The cracking, while not detrimental to the function of the piston, has resulted in concern about the merit of a disc brake piston non-metallic material which can compete with the performance and economics of the steel piston it is designed to replace. Such cracking cannot be tolerated in an application demanding a high degree of reliability over a sustained period of operation. A configuration of piston which can minimize or eliminate such cracking prolongs the useful life of disc brake piston assemblies, provides an increased margin for safety in motor vehicle or other applications, and retains all the advantages of plastic disc brake pistons over metal pistons.
Furthermore, a configuration which can minimize or eliminate such cracking will encourage brake engineers to look more favorably to plastic materials which will satisfy the structural and economic needs of disc brake piston designers.